Plesiosaurs, marine reptiles that thrived in the world's seas when dinosaurs ruled the land, swam much like penguins by using their flippers to "fly" underwater, scientists said Thursday, resolving a debate that began nearly two centuries ago.

Plesiosaurs had four large flippers, and many had remarkably long necks. They lived from about 200 million years ago to 66 million years ago, disappearing in the same mass extinction that doomed the dinosaurs.

Nessie, Scotland's mythical Loch Ness monster, often is portrayed as looking like a plesiosaur.

The researchers conducted a series of computer simulations based on the anatomy of a plesiosaur from 180 million years ago called Meyerasaurus to find the most effective swimming strategy for this body design.

The method that produced the fastest forward speed was flapping the two front flippers up and down in an underwater flying motion similar to penguins and sea turtles.

"What was unexpected was that no matter what motion we simulated for the back flippers, they could not substantially contribute to the plesiosaur's forward motion," said Georgia Institute of Technology computer science professor Greg Turk.

The back flippers were probably used to steer and provide stability, said the researchers, whose work was published in the journal PLOS Computational Biology.

‘Weird and unique creatures’

Plesiosaurs, which ate fish and squid, came in various shapes and sizes, some with shorter necks and others with lengthy ones. Elasmosaurus, for example, was about 46 feet (14 meters) long. Meyerasaurus, unearthed in Germany, measured 10 feet (3 meters) long.

"The plesiosaurs were a highly successful group of large predatory creatures, yet we didn't know how they swam,” Turk said. “Their body plan wasn't an isolated fluke, pardon the pun, because the history of plesiosaurs stretched over 135 million years and dozens of species."

Plesiosaur fossils were first described in 1824. Ever since, scientists have debated how they swam.

"Plesiosaur swimming has remained a mystery for almost 200 years because it is difficult to determine how an extinct animal with a unique body plan moved," said paleontologist Adam Smith of Britain's Nottingham Natural History Museum.

There have been competing hypotheses, with some researchers suggesting plesiosaurs moved their limbs mostly backward and forward, in a rowing motion.

The underwater flying method is unusual because swimming creatures, including most fish and whales, tend to generate thrust using their tails, Smith said.